This invention relates generally to the field of thin film processing such as is used in depositing or removing material from an article and as is often used in the creation of semi-conductor devices. Specifically, the invention relates to the narrower aspect of igniting a plasma in a processing system within which thin film processing is effected by the plasma.
The field of thin film processing broadly encompasses a number of different techniques whereby metallic and non-metallic materials are deposited or removed from an item. This may be accomplished in a variety of ways. One technique involves the use of a plasma of charged particles to effect the processing. Since the popularity of this technique--as well as that of thin film processing in general--has greatly increased in recent years due to the demand for semi-conductor devices, effort has been directed to the actual processing of items or substrates through the utilization of a plasma. In sharp contrast, however, the aspect of actually igniting the plasma in such a system has been relatively ignored. This is perhaps due to the fact that in some applications a plasma can be easily ignited. This is not the case, however, for many thin film processing applications, and is even less the case as processing becomes more exact.
Currently, many thin film processing applications involve the precise processing of one or more items in a very dynamic environment. This dynamic environment is not only unpredictable, it causes fluctuation in a variety of characteristics necessary to cause the gas involved to ignite into a plasma. Thus, while in other applications the plasma can be ignited by merely achieving the necessary conditions, in most thin film processing applications, this is not possible. The dynamic nature of the chamber itself and its unpredictability, make knowing the conditions necessary for ignition extremely difficult from a practical perspective.
In addition, two other aspects exist which explain the difficulty those attempting to ignite a plasma for thin film processing have faced. First, as mentioned, the processing is often very delicate. As semi-conductor manufacturing techniques have been refined, the thickness of the layers within them has decreased to the atomic scale. Thus, tolerances within the processing itself have become much more exacting. While at first glance this would appear to relate only to the operation of the plasma, not to its ignition, such is not the case as the environment during ignition itself can literally ruin the processing desired. Prior to the present invention, however, a technique for igniting a plasma in a processing system which affirmatively minimized undesirable effects has not existed. That those skilled in the art simply did not recognize the teachings of the present invention even though the implementing arts were well known and available is evidenced by the fact that in some systems, literally additional mechanical devices functioning as shutters have been included to protect the item to be processed during an ignition phase.
A second aspect which explains the challenges faced by those skilled in the art, is the fact that processing itself has evolved almost as an eclectic art. While on the one hand the physical and chemical processes involved during thin film processing have been studied and refined from very theoretical bases to produce state of the art devices, still, oftentimes, some of the equipment utilized to actually achieve the thin film processing has been what could be characterized as antiquated equipment and has been applied on what might even be characterized as a trial-and-error basis. This aspect is perhaps best demonstrated by the fact that vital electrical components in many systems are devices originally designed in the 1920's even though greatly improved parallels are well known for application to other fields. Simply put, it may be said that while those skilled in the art of thin film processing might typically have a great degree of skill and technical knowledge in the plasma processes themselves, those so skilled rarely combined such knowledge with equivalent understanding of the high frequency electrical circuitry characteristics as well.
These facts also in part explain why, until the present invention, those skilled in the art have not realized the teachings of this invention even though the implementing arts have long been available and even though those skilled in the art have long felt a need to achieve the objects of the present invention. The preconceptions fostered by those skilled in one field but not another have fostered those in the art to either accomplish ignition of a plasma through different techniques or to effect solutions which in hindsight might appear not well thought out.
Prior to the present invention, several techniques were known to ignite plasmas in general. Primarily, these techniques involve the addition of separate equipment in order to achieve the conditions necessary to ignite the gas into a plasma. An example of these is U.S. Pat. No. 4,906,811 which discloses the addition of separate equipment to locally increase pressure in the gas to effect conditions more conducive to the ignition of the gas into a plasma. Different techniques similarly add separate equipment to create other such drastic changes. U.S. Pat. No. 4,557,819 disclose a separate DC voltage source to momentarily increase the voltage across the gas by about 5,000 volts; U.S. Pat. No. 4,888,088 discloses a separate, lower frequency power source to achieve ignition; and finally, U.S. Pat. No. 4,859,909 discloses the utilization of ion sources or electron sources to prompt the ignition of the gas into a plasma. As mentioned, each of these utilizes separate equipment to effect gross changes under which ignition of the plasma is almost certain to occur. In addition to this published literature, some of those skilled in the art have apparently attempted to automatically manipulate existing electrical circuitry. Some have tried preset values; some have tried sweeping tuning equipment. The techniques used however, have not been accomplished in a manner which demonstrates understanding of the electrical circuitry aspect of such processing systems.